During the production of semiconductor devices it is necessary that certain critical parameters are met. For example, semiconductor device leads must meet parameters on tip-offset, lead skew, and lead coplanarity. To ensure that a semiconductor device has met these parameters they must be inspected after production. This type of inspection often is done manually by assembly line workers. This type of inspection is labor intensive and thus very expensive.
Prior art automated inspection methods use cameras and computer systems to measure lead parameters. A semiconductor device is placed on a testing pedestal, and a digital image of its leads are taken. The computer system uses the image to determine lead parameters. These methods, while much faster than manual methods, require expensive high resolution cameras and expensive corrective optics.
Conventional automated systems are also difficult to calibrate. To calibrate these systems a high tolerance optical calibration gauge is used. The gauge is placed on a test pedestal, and its position is recorded by the computer. This recorded position is used as the reference plane for determining lead parameters such as coplanarity. Unfortunately, the high tolerance gauges required are expensive. Also, if the relative position of the camera and semiconductor device change the recorded lead position will no longer be accurate. Thus extreme care must be taken to insure the camera and semiconductor device pedestal have a fixed relative position. This often requires large and complicated devices that require significant space.
Consequently it is desirable to have a method for measuring lead coplanarity that does not require the use of high resolution cameras and does not require the use of a precise optical calibration gauge for reference plane definition.